Select "CNAME Record" from the "Type" drop-down menu and enter your subdomain as the host (for example, "www" if your domain is www.yourdomain.tld, or "address" if it is address.yourdomain.tld).

Under Value, enter "unbouncepages.com".

Click "Save all changes" and remember that it can take up to 48 hours for newly-created records to take effect. Once your CNAME is ready, you can publish your pages where you want by adding your domain to Unbounce and pointing your landing pages to your custom domain.